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Develop to infective stage In soil

Eggs ingested and hatch

Hatch In soil

Migrate via bloodstream

Larvae - penetrate intestine

Rhabditiform (noninfective) larvae become filarifrom (infective) larvae

Penetrate skin or ingested

Figure 49-64 L i fe cycle o f Ascaris lumbricoides and hookworms

(indirect type of cycle).

Nematodes

Nematodes are elongate, cylindric worms with a well-developed digestive tract, and have separate sexes, with the male being smaller than the female. Most nematodes are diagnosed by finding the characteristic eggs in the stool (see Figures 49-62 to 49-69). The eggs of Ancylostoma duodenale and Necator americanus are essentially identical, so an infection with either species is reported as "hookworm eggs present."

Strongyloides stercoralis is passed in the feces as the noninfective rhabditiform larva (see Figure 49-67). Although hookworm eggs are normally passed in the stool, these eggs will continue to develop and hatch if the stool is left at room temperature for several days. These larvae may be mistaken for those of Strongyloides. Figure 49-68 illustrates the morphologic differences between the rhabditiform larvae of hookworm and Strongybides. Recovery of Strongyloides larvae in duo-ial contents is discussed earlier in this chapter.

"LI"

The technique for recovery of Enterobius vermi-cularis (pinworm) eggs can be seen in Figures 49-4 and 49-69. Eggs of other nematodes are relatively easy to find and differentiate one from another.

Cestodes

The tapeworms have a long, segmented, ribbonlike body that has a special attachment portion, or scolex, at the anterior end (see Figures 49-70 to 49-74). Adult worms inhabit the small intestine; however, humans may be host to either the adult or the larval forms, depending on the species. Cestodes generally require one or more intermediate hosts for the completion of their life cycle (see Figures 49-60 and 49-61). Taenia spp. tapeworm proglottids tend to be longer than wide and contain uterine branches; more than 12 branches per side is T. saginata; less than 12 is T. solium (see Figure 49-71).

Except for Diphyttobothrium latum, tapeworm eggs are embryonated and contain a six-hooked oncosphere (Table 49-21; see also Figure 49-70). Taenia saginata and T. solium cannot be identified to species based on egg morphology; gravid proglottids (see Figure 49-71) or the scolices must be examined.

Hymenolepis nana has an unusual life cycle; ingestion of the egg can lead to the adult worm in humans, thus bypassing the need for an intermediate host (see Figure 49-72). The eggs of H. nana and H. diminuta are very similar; however, H. nana eggs are smaller and have polar filaments, which are present in the space between the oncosphere and the eggshell (see Figure 49-70).

The freshwater fish tapeworm, Diphyttobothrium latum, does not have embryonated eggs. The eggs are operculated, similar to trematode eggs (see Figure 49-70). The proglottids from the adult worm are much wider than long (opposite that seen with the Taenia spp. tapeworms). While the adult worms of H. nana and H. diminuta are relatively small, Taenia spp. range from about 12 to 15 feet, and D. latum can reach 30 feet in length.

Trematodes

Humans acquire most fluke infections by ingesting the encysted metacercariae (see Figure 49-75). Most trematodes have operculated eggs, which are best recovered by the sedimentation concentration technique rather than the flotation method. Often, careful measurements may be required for egg identification (see Figure 49-76). Some of the smaller eggs maybe missed unless the high dry objective (40x) is used for microscopic examination.

Paragonimus spp. eggs are found not only in sputum but also in stool (sputum swallowed). These eggs are similar in size and shape to those of D. latum.

Figure 49-65 A, Immature hookworm egg. B, Embryonated hookworm egg. C, 1Yichostrongylus orientalis, immature egg. D, Strongybides stercorals, rhabditiform larva (200 pm). E, Enterobius vermicutaris egg. F, TYichuris trichiura egg. G, Ascaris lumbricoides, fertilized egg. H, A lumbricoides, fertilized egg, decorticate. I,A. lumbricoides, unfertilized egg. J, A. lumbricoides, unfertilized egg, decorticate.

50 jim

Figure 49-65 A, Immature hookworm egg. B, Embryonated hookworm egg. C, 1Yichostrongylus orientalis, immature egg. D, Strongybides stercorals, rhabditiform larva (200 pm). E, Enterobius vermicutaris egg. F, TYichuris trichiura egg. G, Ascaris lumbricoides, fertilized egg. H, A lumbricoides, fertilized egg, decorticate. I,A. lumbricoides, unfertilized egg. J, A. lumbricoides, unfertilized egg, decorticate.

Figure 49-67 Strongybides stercorals rhabditiform larva, iodine stain.

Figure 49-66 Hookworm egg, iodine stain.

Figure 49-67 Strongybides stercorals rhabditiform larva, iodine stain.

Schistosome eggs are relatively easy to identify and have terminal (Schistosoma haematobium), lateral (S. mansoni), or small lateral (S.japonicum) spines (see Figure 49-76). These eggs are nonoperculated; specific procedures for egg recovery and identification are found in earlier section on the egg hatching procedure.

Chapter 49 Laboratory Methods for Diagnosis of Parasitic Infections 611 ABC

be Atv.bc be

Figure 49-68 Rhabditiform larvae. A, Strongyloides. B, Hookworm. C, Trichostrongylus. be. Buccal cavity; cb, beadlike swelling of caudal tip; es, esophagus; gp, genital primordia. (Illustration by Nobuko Kitamura.)

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Bacterial Vaginosis Facts

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